As an agricultural combine is propelled through a field, it cuts the standing crop and feeds the cut crop to a threshing cylinder which beats the crop material to initially separate most of the grain from the straw which is discharged out of the rear of the combine. The separated grain containing chaff and other foreign material is usually fed to a cleaning shoe which conventionally includes a plurality of vertically spaced oscillatory sieves that separate the grain from the chaff and feed the clean grain forward while a stream of chaffer air is directed rearwardly through the sieves and blows the chaff and lighter foreign material out of the rear of the combine. The grain falling through the front end of the sieves is collected and moved up a clean grain elevator to a storage area, while the material falling through the rear end of the cleaning shoe is a fluffy mixture of grain and mature vegetation on which it grows, such mixture being termed "tailings" which are usually collected and delivered by a tailings elevator to be rethreshed. In the case of wheat, barley and similar crops, tailings include the grain which remains attached to the fragments of the heads, while in the case of corn, tailings include kernels which remain attached to pieces of the cob.
The volume of tailings being recycled depends upon the efficiency of the threshing cylinder and the cleaning shoe, and such efficiency is determined by such factors as clearance between the threshing cylinder and the concave, the speed of the threshing cylinder, the sieve opening, the volume of chaffer air, and the rate at which crop is fed into the combine--the last factor depending upon speed of the combine and is easily adjusted by the combine operator. Windows in the tailings return are known to permit the combine operator to observe the volume of tailings. If the tailings are not sufficiently dry, the combine operator may desire to delay threshing for several days. If volume of tailings is excessive, the combine operator may decide that the threshing mechanism and cleaning shoe are excessively loaded so that combine speed should be reduced, and thus the rate at which the crop is fed into the combine. Alternatively he may increase the sieve opening or the chaffer air for the purpose of reducing the volume of tailings. However, access from the operator's station to such window in the tailings return presents a problem, and even if such access is possible, continuous visual checking of the tailings diverts the attention of the operator from steering and operation of the combine.
U.S. Pat. No. 3,515,144 to Morrison discloses a tailings monitor having a pivoted sensing element at the discharge end of the tailings elevator which is deflected in response to the weight of tailings delivered by the elevator and an electrical circuit associated with the sensing element which closes a switch to light an indicator lamp when the quantity of tailings exceeds a predetermined amount. However, the response of such monitor having a deflectable weight-responsive sensing element is affected by both the density of the tailings and the velocity at which such tailings are discharged from the tailings return conveyor with the result that the monitor will not always accurately indicate the volume of tailings which are not granular but rather have stalks, leaves and heads of mature crops and are "fluffy." Further, such deflectable-element tailings sensing means is difficult to calibrate.